In established primary hypertension cardiac output is normal, while peripheral vascular resistance is increased. Many factors may contribute to the elevated vascular resistance. Recently, it became obvious that the endothelium can affect vascular tone by releasing an endothelium-derived relaxing factor(s); endothelium-dependent relaxations appear to be reduced in arteries from rats with genetic hypertension. Under certain circumstances (e.g. anoxia) the endothelium can also release a constrictor substance(s). Preliminary experiments suggest that acetylcholine and serotonin can release a vasoconstrictor material from the aortic endothelium of genetically hypertensive rats (SHR) but not from normotensive controls (WKY). The major purpose of the present proposal is to explore the importance of the endothelium-derived contracting factor(s) in spontaneously hypertensive rats. Experiments will be performed on isolated aortas (and in some cases other arteries and veins), with and without endothelium, of SHR and WKY. Responses of vascular smooth muscle will be recorded as isometric tension. To demonstrate the release of the contracting factor(s) from the endothelium by acetylcholine and other potential releasers (neurohumoral mediators involved in hypertension, platelets, inhibitors of Na+,K+ ATPase, hypoxia, transmural pressure) we will use layered preparations ("sandwich preparations") and a bioassay apparatus. To determine the nature of the endothelium-derived contracting factor(s) will require examination of the metabolism of arachidonic acid, of the involvement of other neurohumoral mediators, and of the role of oxygen-derived free radicals. To determine the cellular mechanism of action of the endothelium-derived contracting factor(s) on vascular smooth muscle will require measurement of membrane potential and pharmacological interference with Na+,K+ ATPase, the metabolism of arachidonic acid, and Ca2+ movements. The importance of chronic exposure to high blood pressure and of aging for the occurrence of endothelium-dependent contractions will be examined. Finally, we will determine whether endothelium-dependent contractions occur in blood vessels of rats and dogs with secondary hypertension. A disturbed endothelial function could contribute to the elevated peripheral resistance of hypertension and complications such as myocardial infarction and stroke. It is expected that the knowledge gained from the proposed research will provide new insight concerning the role of the endothelium as a source of vasoconstrictor signals.